Method for removing filtercake

ABSTRACT

A method for removing filtercake from a subterranean borehole comprising drilling the borehole with a drilling fluid that includes additives to form a filtercake having an oxidation-degradable component, preferably a polymer. The filtercake is contacted with a precursor that forms an oxidizing agent at downhole temperatures to degrade the polymers within the filtercake. Alternatively one or more catalysts for activating the precursor of an oxidizing agent is embedded within the filtercake. The catalyst activates the precursor as the precursor is flushed over the filtercake and remains in contact with the filtercake to form an oxidizing agent to degrade the oxidation-degradable agent and decompose the filtercake. The remaining particles of decomposed filtercake are then flushed away.

FIELD OF THE INVENTION

The present invention relates to a method for removing filtercake from asubterranean borehole, particularly to a method for filtercake removalby contacting the filtercake with a precursor to an oxidizing agent andmore particularly to a method of filtercake removal in which a catalystis included in the formation of the filtercake forming fluid toaccelerate the conversion of the precursor to the oxidizing agent.

BACKGROUND OF THE INVENTION

The walls of oil and gas formations are exposed during the process ofdrilling a borehole. The successful completion of a wellbore requiresthe deposit of a low-permeable filtercake on the walls of the wellboreto seal the permeable formation exposed by the drilling bit. Afiltercake can limit drilling fluid losses from the wellbore and protectthe natural formation from possible damage by the fluids permeating intothe wellbore. Solids in the drilling fluid may also damage theformation, particularly drilling fines. The suspension of fine particlesthat enters the formation while the cake is being established is knownas "mud spurt" and the liquid that enters subsequently is known as"filtrate". Both filtration rate and mud spurt must be minimized whenpenetrating potentially productive formations because productivity maybe reduced by any one of the following: the swelling of clays in theformation when they come in contact with the filtrate; particlestransported into the pores of the formation that plug flow channels andgreatly reduce the permeability of the rock; and the pressure of somereservoirs that is not great enough to drive all of the aqueous filtrateout of the pores of the rock when the well is brought into production.

Contamination of the formation with drill solids in the fluids can beavoided by formulating the drilling fluid with other soluble solids thatcan be incorporated in the filtercake and dissolved at a later time,thereby diluting the effect of the insoluble solids. Sized salt solids,in salt saturated solutions, and finely ground calcium carbonate areexamples of solids purposely added to the drilling, workover, orcompletion fluids to form a filtercake that can later be partiallydissolved by aqueous or acid flushes.

For a filtercake to form, the drilling fluid must contain some particlesof a size only slightly smaller than the pore openings of the formation.These particles are known as bridging particles and are trapped insurface pores, thereby forming a bridge over the formation pores.Filtercake building fluids can also contain polymers for suspension ofsolids and for reducing liquid loss through the filtercake byencapsulating the bridging particles. These can be either natural orsynthetic polymers. The polymers can include one polymer such as xanthanselected for its Theological properties and a second polymer, a starchfor example, selected for reduction of fluid loss.

At completion of the drilling or other well servicing, the filtercakemust be removed to allow production of the formation fluids or bondingof cement to the formation at the completion stage. Removal of thedeposited filtercake should be as complete as possible to recoverpermeability within the formation.

Previous chemical treatments for filtercake removal have employed anacid to dissolve carbonates and/or hydrolyze polysaccharide polymers.Dobson, Jr. et al., U.S. Pat. No. 5,607,905, reveal a process forenhancing the removal of filtercake by the use of inorganic peroxides asoxidizing agents. The process disclosed in the '905 patent incorporatesalkaline earth metal peroxides, zinc peroxides or a mixtures thereofwithin the filtercake as an integral component thereof and then contactsthe filtercake with an acidic solution. Weir et al., U.S. Pat. No.1,984,668, disclose a method of cleaning the walls of mudded boreholes.A first reagent is included in the mud-coating of the borehole.Subsequently, the mud-coating is impregnated with a second reagent thatreacts with the first reagent. The first reagent is generally acarbonate. The second reagent is generally an acid that reacts with thecarbonate to form carbon dioxide gas.

Tjon-Joe-Pin et al., U.S. Pat. No. 5,247,995, disclose a method forremoving a polysaccharide-containing filtercake formed on surfacesduring fracturing with a viscosified fluid. The '995 method disclosesremoval of the filtercake by pumping downhole an aqueous fluidcontaining enzymes that degrade polysaccharide. The use of non-metallicpersulfates as an oxidant in filtercake removal systems is alsodisclosed.

A composition and method of removing polymeric material from a formationis found in Hanlon, U.S. Pat. No. 4,609,475. The method disclosedcomprises contacting the formation with an oxidizing agent andcarboxylic acid. A source said to be effective in promoting thedecomposition of the oxidizing agent is added to the aqueous solutioncontaining the oxidizing agent.

The addition of an alkali metal or alkaline earth metal salt ofhypochlorous acid, or a chlorinated isocyanurate is disclosed inLangemeier et al., U.S. Pat. No. 4,941,537, for the reduction ofviscosity of aqueous fluids thickened with a tertiary aminopolygalactomannan.

Methods of filtercake removal that incorporate a solid oxidizing agentprecursor in the filtercake are not totally insoluble at thetemperatures of application so that some oxidizing materials arereleased with premature degrading of the polymer. Also, the materialsare only applicable in high pH fluids, thereby limiting their use.Finally, these compositions cannot be used in formulations withsignificant amounts of reducing agents, such as iron control agents,oxygen scavengers, or bromide or formate brines.

A problem associated with the use of acid to activate a precursor infield use is that sufficient acid does not always contact all parts ofthe filtercake. Sometimes the acid must be "weighted up" (formulatedwith high weight brines to increase fluid density). Thus the actual acidcontent is low. As the acid is reacting with the carbonates in thefiltercake, a stoichiometric amount of acid is required, and this volumecan be greater than the actual exposed wellbore volume; the acid must becontinuously replaced. In some cases, the acid initially dissolves asmall region of the filtercake, because of the vastly increased fluidloss of this region, the remaining acid flows to this small region andout into the formation. What is needed is a method that attacks thefiltercake in a manner which does not completely remove the filtercakein a small region and does not require large stoichiometric amounts ofagent.

In the drilling and completion of a well, several operations after thedrilling process are often required before production can begin. Theseoperations require the displacement of the drilling fluid withoutdegradation or removal of the filtercake. What is needed, therefore, isa method for removal of filtercake that does not depend on the presenceof a drilling fluid.

In the case of horizontal open hole drilling of unconsolidatedformations, it is desirable to gravel pack the wellbore after drillingthe zone but before the filtercake is completely removed. The problem isthat the act of gravel packing the wellbore annulus further limits acidcontact with the filtercake, as it both reduces the physical volume ofacid in the zone and blocks its flow.

Consequently, there remains the need for a filtercake removal technologythat does not depend solely on acid hydrolysis for removal of thefiltercake. And there is an additional need to provide a method foroxidizing filtercake materials in which the oxidizing agent is notactivated prematurely, i.e. at ambient temperatures, so that theoxidizing agent works in concert with drilling fluid formulations thathave easily oxidized materials.

SUMMARY OF THE INVENTION

In the method of this invention, the removal a filtercake from asubterranean borehole comprises the steps of drilling the borehole witha drilling fluid to form a filtercake comprising an oxidation degradablecomponent, contacting the filtercake with an organic hydroperoxide,allowing the organic hydroperoxide to remain at the downhole temperaturefor a period of time effective to form an oxidizing agent to degrade theoxidation-degradable agent and decompose the filtercake and flushingaway the decomposed filtercake. The oxidation-degradable component canbe a polymer.

Preferably, the polymer comprises starch, cellulose or xanthan. In oneaspect, organic hydroperoxide is thermally activatable at the downholetemperature of the filtercake In a preferred embodiment, the organichydroperoxide is t-butylhydroperoxide and the borehole temperature is atleast about 80° C. Alternatively, the organic hydroperoxide is selectedfrom the group consisting of cumene hydroperoxide, t-butyldihydroperxoide, and amylhydroperoxide, and the borehole temperature isat least about 80° C.

In one embodiment, an activating agent for the organic hydroperoxide isflushed over the filtercake prior to the organic hydroperoxide flush. Inan alternative embodiment, the filtercake comprises an activating agentfor activating the organic hydroperoxide. Preferably, the activatingagent comprises an ascorbic acid. Alternatively, the activating agentcomprises a tertiary amine. In another embodiment, activating agent isdiethylaminoisopropanol. Alternatively, the tertiary amine comprisestriethanolamine. In one aspect, the activating agent is a polymercomprising pendant tertiary amine moieties. The oxidation-degradablepolymer is functionalized with the amine moieties. An additionalactivating agent selected from the group consisting of copper saltsolution cobolt salt solution, iron salt solution and chromium saltsolution is flushed over the filtercake.

In one preferred method for removing filtercake from a subterraneanborehole, the steps for removal of the filtercake comprise drilling theborehole with a drilling fluid to form a filtercake comprising anoxidation-degradable component and an activating agent for activating aprecursor of an oxidizing agent, wherein the filtercake is free of theprecursor, contacting the filtercake with a precursor of oxidizingagent, allowing the precursor to remain in contact with the filtercaketo form an oxidizing agent to degrade the oxidation-degradable agent anddecompose the filtercake, and flushing away the decomposed filtercake.Preferably, the oxidation-degradable component is a polymer. The polymercan comprise starch, cellulose or xanthan. The activating agent cancomprise a tertiary amine. Preferably, the tertiary amine comprisestriethanolamine. Alternatively, the activating agent is a polymercomprising pendant tertiary amine moieties. The oxidation-degradablepolymer can be functionalized with the amine moieties. In one aspect,the precursor is selected from alkali metal and ammonium salts ofpersulfates, perborates, permangamates and percarbonates. Alternatively,the precursor is selected from the group consisting of t-butylhydroperoxide, cumene hydroperoxide, t-butyl dihydroperxoide, andamylhydroperoxide. Preferably, additional the steps for this preferredmethod include: installing gravel pack screens and tool assemblies intothe borehole prior to the step of contacting the filtercake with theprecursor, determining fluid losses and adding sand in a nonviscosifiedcarrier to the borehole.

DETAILED DESCRIPTION OF THE INVENTION

In the method of this invention, filtercake formed on the walls of asubterranean borehole is removed by contacting the filtercake with aprecursor to an oxidizing agent. In one preferred embodiment, a catalystis included in the filtercake-forming fluid to accelerate the conversionof the precursor to the oxidizing agent. Filtercakes are tough, almostwater insoluble coatings that reduce permeability of formation walls.Formed during the drilling stage to limit losses from the wellbore andprotect the formation from possible damage by fluids and solids withinthe wellbore, filtercake layers must be removed so that the formationwall is restored to its natural permeability to allow for hydrocarbonproduction or cementing.

Filtercakes can be formed with polymers that encapsulate particles orsolids which form a bridge over the pores of the formation. Drillingfluids as well as any bridging agents and polymers contained within thedrilling fluid are well known in the art. In the method of thisinvention, the preferred drilling fluid is a brine, more preferably, acalcium bromide brine with additives, preferably polymers, salts,carbonates and other soluble solids. An example of a suitable drillingfluid according to the present invention comprises a calcium bromidebrine with a cationic starch, a cross-linked, non-cationic starch,sodium thiosulfate, magnesium oxide, and xanthan biopolymer. Thebridging agent within the drilling fluid may include either watersoluble or acid soluble materials, for example: sized salt solids insalt saturated solutions, finely ground carbonates found in limestone,marble, or dolomite; or oil soluble such as resins, waxes and the like.Polymers can be either natural or synthetic polymers. Preferred polymerscomprise starches and xanthans. Alternatively, the polymer can beselected from starch derivatives, cellulose derivatives and biopolymerssuch as hydroxypropyl starch, hydroxyethyl starch, carboxymethyl starchand their corresponding crosslinked derivatives; carboxymethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methylcellulose, dihydroxypropyl cellulose, and their correspondingcrosslinked derivatives; xanthan gum, gellan gum, welan and the like.

In the drilling and completion of a well, several operations may berequired after the drilling process before the well is produced. Forthese stages it is usually desirable to maintain the filtercake on thewellbore thereby limiting fluid losses to the formation. Immediatedegradation or removal of the filtercake is not desired. The drilling orcompletion fluid is displaced without removing the filtercake. Knownprocedures which are designed to achieve this displacement are believedto be compatible with the invention. In other situations, such as thecementing of a cased hole, the drilling fluid and the filtercake areremoved together. The preferred practice is to completely displace thedrilling fluid first, and then treat to remove the filtercake.

According to the filtercake removal method of this invention, afiltercake is formed with an oxidation-degradable component, preferablya polymer as described above. Oxidizing agents, or preferably precursorsto oxidizing agents, attack and decompose the oxidation-degradablecomponent. In one embodiment of the method of filtercake removal, thefiltercake is treated or flushed with a precursor to an oxidizing agent,for example an organic peroxide or hydroperoxide. Preferably theprecursor remains in contact with the filtercake as a soak for a timeperiod of from one to six or more hours. Preferably the organic peroxideor hydroperoxide is soluble or dispersible in aqueous mixtures andrelatively inert at ambient temperatures. Because of these properties,the organic peroxide or hydroperoxide can be included in the finaltreating flush of weighted brines with little oxidation of the halide tothe free element. At downhole temperatures, however, the oxidativeactivity increases. Organic peroxides and hydroperoxides are selectedfor the characteristic of increasing reactivity with increasingtemperature. Commercially, they are described as having a declininghalf-life at various increasing temperatures that correspond to thisincreasing oxidative activity. Consequently the preferred precursor toan oxidizing agent is selected so that an oxidizer is rapidly generatedat reservoir formation temperatures although stable in storage atambient conditions.

Preferred precursors having such properties compriset-butylhydroperoxide, tertiary dibutyl hydroperoxide, cumenehydroperoxide, or amyl-hydroperoxide, especially t-butylhydroperoxide.They can be used as precursors in dilute concentrations. Some chloritesolutions are also stable until activated at particular temperatures,including chlorinated isocyanurates, such as, for example,dichloro-s-triazine-2,4,6-trione or dichloroisocyanuric acid. Thus theformation temperature activates the precursors in the degradation offiltercake. Preferably, the formation or borehole temperature is atleast 80° C.

In another embodiment of this invention, the filtercake is flushed witha fluid containing a catalyst prior to the soak with a precursor. Thecatalyst functions as an activating agent for forming the oxidizingagent from the precursor. Various activating agents can be selected tocatalyze the decomposition of precursors. Preferably, specific catalystsare used with specific precursors, for example, tertiary amines andascorbic acid act as catalysts to degrade t-butylhydroperoxide. In oneaspect, the catalyst is included in the drilling fluid displacementstage and the oxidizing precursors are then included in a later flush orflushes intended to dissolve and disperse the filtercake. The laterflushes can include surfactants and other materials designed to displacethe polymeric components from the surfaces of the drill solid materials,such as, for example, lactose or methyl glycoside.

Oxidizing agents formed from the precursors attack polymersencapsulating the bridging particles and break down the filtercake byremoving the polymeric coat, with the effect that the residualfiltercake solids are no longer bridged or bonded by polymers.Preferably, the polymer is completely dissolved so that the carbonateparticles do not bond to each other. These bridging solids can theneasily be dispersed as individual particles, not clustered films. Theremoval of the filtercake particles can then occur either particle byparticle as production fluid flows out of the formation into thewellbore, or by the addition of a solubilizing flush to dissolve thebridging solids. Although no acid is required, since the filtercakeparticles are small enough to flow through as production fluids flowinto the bore, an acid flush is preferably injected into the bore todissolve the carbonate bridging component. The acid can be appliedimmediately, or alternatively, flushed over the filtercake at a latertime. Alternatively, the acid can be generated in place, for example, bythe decomposition of a polyglycolic acid.

In an alternative embodiment, a catalyst or activating agent for theprecursor is incorporated into the filtercake. The catalyst is includedin the drilling fluid along with other filtercake building materials todeposit into the filtercake as it is formed. Oxidation of the filtercakeoccurs when a precursor is flushed over the filtercake and decomposes,upon activation by the catalyst, into an oxidizing agent. Even thoughthe catalyst is within the filtercake, no oxidative attack on thepolymers of the filtercake takes place until the filtercake is contactedwith the oxidizing precursor. Specific catalysts are used with specificprecursors as described above, preferably tertiary amines or ascorbicacid with t-butylhydroperoxide. Preferably, the catalyst is chemicallybonded as an adduct with targeted polymers, tertiary amines as adductson starches or xantham for example. For that reason, the decompositionof the oxidizing precursor occurs locally, in the molecular region wherethe oxidative attack of the polymer is desired.

In one aspect, the catalyst is a tertiary amine or aminoalkanol, suchas, for example, triethanolamine, diethylaminoisopropanol, or the like.In another embodiment, the catalytic materials are polymeric and becomepart of the polymeric component of the filtercake, as functionalmoieties dependent from the polymeric backbone e.g., thereby limitingcatalyst loss with brine or fluids that are lost to the formation fromthe filtercake building fluid. Preferably, the catalytic moiety can be acomponent of one or any combination of the polymers chosen for use inthe formation. The activating agent or catalyst can be a polymercomprising pendant tertiary amine moieties. One preferred activatingagent comprises an amine adduct to a starch or viscosifying polymer. Ina preferred embodiment, the catalyst is a diethylaminoisopropanol adductto either a starch or xanthan.

When the catalyst or activating agent is incorporated within thefiltercake, decomposition of the filtercake begins as the precursor isplaced in contact with the filtercake and then remains in contact as asoak until the filtercake is decomposed. Preferably, the precursor isone that is stable at ambient temperatures and decomposes to theoxidizing agent at downhole temperatures, between 40° C. and 120° C. Theoxidizing agent breaks down the polymeric coating on the carbonateparticles of the filtercake thereby destroying the bridging feature ofthe filtercake. The particles can be further reduced or dissolved by theuse of a separate acid wash after the precursor soak.

In a preferred method of filtercake removal, a well is drilled using adrilling fluid that includes an amine-substituted starch and anamine-substituted xanthan thickening agent. Preferably the amine isdiethylaminoisopropanol. Alternatively, a polymer containing tertiaryand quaternary functional groups, such as a copolymer ofpolyethylenimine and ethylene oxide may be included. The preferredamounts of polymer are 0.14 g/l to 28.6 g/l. The fluid preferablycontains about 85.7 g/l of sized carbonate, as well as drill solidsresulting from the drilling operations. Other additives, such as gelstabilizers, clay treating additives, pH control agents, anti-oxidants,lubricants, non-emulsifying agents, iron control agents and the like canbe included as desired.

Following the drilling of a well, when fluid losses are acceptable forthe proposed pumping pressures, gravel or sand packing can begin. Firstthe drill-in fluid is displaced with a first clear fluid, which isotherwise similar to the drilling fluid. The wellbore is maintained in aslightly overbalanced state. Gravel pack screens and tool assemblies arerun into the bore. During this stage, it is desirable to maintain thefiltercake with as little fluid loss to the production formation aspossible. Following displacement of the drilling fluid, the well isgravel packed. In a preferred procedure, the gravel, preferably sizedsand, about 20-30 U.S. mesh, is placed into a nonviscosified carrier,such as a brine. As the low viscosity fluid cannot transport asignificant amount of solids, the concentrations are usually about 60g/l to 360 g/l of sand per gallon of fluid and pump rates approach 1m3/min. The hydrostatic overbalance that arises from the pumpingpressure necessary to achieve these rates is desirable since theoverbalance holds the filtercake in place. An oxidative precursor,preferably t-butyl hydroperoxide, can be added during the sand packing.A preferred practice starts with small concentrations, approximately0.1-0.5%, and increases the concentration of the precursor if there areno fluid loss responses.

Once the gravel pack sand is in place, it becomes more difficult toinsure contact of the filtercake with acidic or other flushes. Treatingfluids, particularly acids, tend to dissolve all of the filtercake insmall areas so that the remaining treating fluid leaks off, or leaves,the wellbore at those points. In one aspect of the present invention,the problem of a flush completely dissolving a small area of thefiltercake is overcome by including the activator in the filtercake. Theprecursor is flushed in at a later time. Alternatively, the precursor ofthe oxidizer is included in the gravel pack carrier fluid itself,combining two treatment steps with resultant savings in rig time. Inanother aspect of the invention, after the drill zone is completelyfilled with sand and flushed with a precusor, the gravel pack can befilled with an acid-containing fluid to dissolve the carbonate.

Prior to the acid flush and after the gravel pack is in place, the firstreplacement fluid is usually displaced with a spacer (buffer) or seriesof spacers, so that a clear fluid is left in the bore. In an alternativemethod, this fluid can contain about 0.014 g/l to 2.86 g/l by weight ofa copper salt. The copper ions, with a strong affinity for tertiaryamines, are retained in the filtercake and function as an additionalactivator to catalyze the breakdown of some oxidizing agents,particularly organic peroxides and hydroperoxides. Alternatively,ascorbic acid is used as the activating agent for the precursor.Ascorbic acid with the addition of the copper salt will rapidly reducethe precursor to an oxidizing agent. Other metal ions, such as cobalt,nickel, iron, and chromium could be substituted for the copper. If fluidlosses are too great during this step, additional sized carbonate can beinjected to reduce such losses.

The additional benefit of the copper catalytic effect is important attemperatures below 60° C. as the amines in the filtercake can catalyzethe decomposition of many oxidizing precursors, including sodiumpersulfate and t-butyl hydroperoxide more effectively. Amines alone withno additional catalyst are sufficient above this bottom holetemperature.

Interaction of the oxidative precursors and the catalysts embedded inthe filtercake generates the oxidation-degradable agent needed todecompose the polymers of the filtercake. The residual filtercakesolids, no longer bridged or bonded by polymer, can be easily dispersed.The removal of the particles can then be by either particle by particleas the well is flowed in the production direction, or by a solubilizingflush such as an acid injected to dissolve carbonate. The acid can beimmediately applied, squeezed at a later time, or generated in place,such as by decomposition of polyglycolic acid.

EXAMPLES

Drill in fluid formulations with a Specific Gravity (SG) equaling 1.44based upon a CaBr₂ based brine were used for the tests.

Formulations

Drill-in Fluid Based Upon CaBr₂

    ______________________________________                                                          Quantities in grams/liter                                                     Test #                                                      Component           1        2                                                ______________________________________                                        Water               424.9    424.9                                            CaBr.sub.2  (SG = 1.70) brine)                                                                    920.9    920.9                                            Cationic Starch     0        13.7                                             Crosslinked, Non-Cationic starch                                                                  13.7     0                                                (FL7+)                                                                        Sodium Thiosulfate  2.86     2.86                                             Magnesium Oxide     2.86     2.86                                             Xanthan biopolymer  3.14     3.14                                             Sized Marble powder #1                                                                            16       16                                               Sized Marble powder #2                                                                            16       16                                               Sized Marble powder #3                                                                            2.86     2.86                                             Tertiary Amine polymer, #1                                                                        0        0.29                                             Tertiary Amine polymer, #2                                                                        0        0.29                                             Xanthan biopolymer  3.14     3.14                                             ______________________________________                                         Sized Marble powder #1 (3μ to 400μ)                                     Sized Marble powder #2 (1μ to 36μ)                                      Sized Marble powder #3 (1μ to 4μ)                                       Tertiary amine polymer #1, Polyethyleneimine copolymer with ethylene          diglycol                                                                      Tertiary amine polymer #2, Polyethyleneimine copolymer with                   epichlorohydrin                                                          

The CaBr₂ brine was a stock commercial solution marketed by TETRATechnologies. The cationic starch used was also commercially availablefrom TETRA Technologies as TETRA HPS. It is an epichlorohydrincrosslinked, pregelatinized corn starch which has been reacted withdiethanolamino-methane. Levels of substitution are about 0.20 to 0.40moles amine per mole amylose. The non-cationic starch was commerciallyobtained as FL 7 PLUS, a trademarked product of TBC-Brinadd. It is anepichlorohydrin crosslinked starch which can be substituted withpropylene oxide. The sodium thiosulfate and magnesium oxide were USPgrade. The xanthan biopolymer is available from several suppliers, thatused was supplied by Drilling Specialties. The sized marble powders areavailable from TETRA Technologies under the trade designation TETRACarb-Prime, TETRA Carb-Fine, and TETRA Carb-Ultra, respectively. Thetertiary amine polymer #1 was a homopolymer of ethylene-imine with about40% of the available nitrogen reacted with ethylene oxide. Molecularweight was 60,000-80,000. Tertiary amine polymer #2 was a homopolymer ofethylene-imine with about 40% of the available nitrogen reacted withepichlorohydrin. Molecular weight was about 40,000.

This mixing procedure was followed for all laboratory tests. Afteradding the starch, and before adding the next ingredients, the mixturewas sheared with a high shear (Silversen type) mixer for 30 seconds, andthen mixed at 500 RPM using a low shear Servodyne unit for 30 minutes.This is intended to simulate mixing with a high shear centrifugal pump,and then slow mechanical rolling of a field mixing unit. This mixingprocedure was repeated after the addition of the next three ingredients,the thiosulfate, magnesium oxide and xanthan. A third mixing for 30minutes was run after adding the carbonates and tertiary amines.

Rheological properties were then measured (heating only the sample usedfor testing to 49° C.), and the samples were "hot-rolled" at 93° C. in aBaroid roller oven for 48 hours. Following the `hot rolling`, theTheological properties were again tested, and the samples were thentested for "filtercake removal" in the following manner.

The permeability of a 5μ Aloxite disk was first determined in bothdirections of flow. This test was run at 35 kPa and 21° C. Next, afiltercake was built using a standard high temperature, high pressurecell (HTHP cell). Fann Instruments and OFI, both in Houston Tex., marketthis equipment. The 5μ Aloxite disk was used as the filtering medium andthe chamber was run upside down so that any solids separating from thedrill-in fluid might fall to the bottom and not become part of thefiltercake. The cell itself was filled with the test drill-in fluid anda higher density fluid was run on bottom from a reservoir attached tothe cell, so that the drill-in fluid might be displaced upward.

The test was run at 93° C. for 39 hours, with a squeeze pressure of3,500 kPa applied to the fluid. The filtrate was collected and measuredduring this time. The objective for producing a filtercake was to allowan initial spurt fluid loss as the filtercake is building, but then havea rapid decline as the filtercake limits further fluid loss. At the endof the cake building time, the cell was cooled and pressure released.Remaining fluid was drained from the cell and the filtercake which hadbeen formed was examined.

Following the visual examination, the chemical treatment for removal ofthe filtercake and recovery of the permeability of the Aloxite disk wasperformed. This part of the testing was run in the normal direction,with the disk (and filtercake on it) at the cell bottom and the treatingfluid carefully poured in on top of it. This fluid was injected in thesame direction as the drill-in fluid to simulate the injection of aclean-up fluid in field practice. Consequently the permeabilitydetermined in this direction is called the recovered injectionpermeability. The test is repeated in the opposite direction, again at35 kPa and ambient temperature. This flow was in the productiondirection of an actual well and is called the recovered productionpermeability.

Test 1

A filtercake as described above was prepared using formulation #1 at 38°C. for heat aging and permeability testing. An initial soak of 3 hourswas tried using 2% by weight of tertiary-butyl hydroperoxide and 0.3% ofan ethoxylated nonylphenol surfactant. This soak was run at roomtemperature and 3500 kPa differential pressure. After 3 hours, less than4 ml of fluid filtrate had been produced and the injection permeabilitywas essentially zero. The cell was loaded again with 2% t-butylhydroperoxide and the 3 hour test repeated. No additional breakthroughof fluid was noted, and permeability, both injection and productiondirections was less than 2%.

Test 2

A filtercake and test similar in all respects to Test I above, exceptthat the temperature of the test was 93° C. In this case the treatingfluid broke through in 45 minutes, flowing at about 1 ml/min. In about15 minutes additional time, this rate had increased to about 5 ml/ min.and the test was terminated after 80 ml of the treating fluid had beendisplaced through the cell. The cell was allowed to cool and pressurewas released. The filtercake was visually inspected and found to becomposed of discrete carbonate particles with no evidence of starches orpolymers. A recovered permeability test was run in the injectiondirection, with 34% recovered permeability. A recovered permeabilitytest was run in the production direction, with 67% of the originalpermeability recovered.

Following this step and prior to an acid flush, 50 ml of an iron controlagent (2-thioethanol) and 0.3% surfactant were flushed through in theinjection direction. This was done as actual field practice would callfor a spacer fluid to separate the oxidizing agent stages from thehydrochloric or hydrobromic acids, which are reducing in nature. Anotherpurpose of such a stage is to complex any Fe⁺⁺⁺ that may have beengenerated. Obviously a wide variety of other agents could be used.

After this buffer or spacer stage, a 5% solution of HCl in 1.32 g/mlCaBr₂ was poured into the cell. The cell was sealed and pressurized to3,500 kPa, which was intended to simulate the spotting (but notinjection) of acid in a balanced hydrostatic condition. After 3 hours,the bottom valve was opened and the acid flushed from the cell. It wasnoted that the time of this acid soak was one half that of the acidsoaks in Tests 6 & 7 below, and only one acid soak was required. Therecovered permeability was 91% in the production direction and 95% inthe injection direction.

Test 3

A test similar to Test 1 was run, except in this case, formulation #2was used; this included a cationic starch as well as amine polymers.Small amounts of fluid broke through after about 2 hours. The totalfluid loss was about 30 ml during the first three hour test run. Afterthe first 3 hour run, the cell was opened, emptied of treating solution,and a fresh 60 ml of treating solution added. Again, slow fluid loss wasnoted at 72.5 m/Pa and 38° C. over the next 3 hours resulting in a totalof 30 ml displaced from the cell. Visual inspection of the filtercakeshowed that a top layer of the polymer had been dissolved, though notcompletely through. This in contrast to Test 1 that showed no suchpolymer attack. Permeabilities were less than 2% in both directions.Again, an iron control agent was added as in Test 2. Following thisstep, the cell was loaded with a 5% HCl in 1.34 g/ml CaBr₂ brine andallowed to soak for 6 hrs. Recovered permeability in the injectiondirection was 4%. After a second, similar 6 hour acid soak, recoveredpermeabilities were 77% in the production direction and 72% in theinjection direction.

Test 4

A test similar to Test 1 using formulation #1, except that heat aging,filtercake build-up and treatment soaks were preformed at 65° C. After 3hours, no significant leak-off of fluid had been observed. The testchamber was refilled with a fresh 2.3% solution and the test run foranother 3 hours. During the final hour of this test, fluid began to leakoff, amounting to about 50 ml at the end of the test. The filtercake waspowdery on top and the polymer appeared to be completely removed.Permeabilities were run in both directions, with the recovered injectionpermeability being 3% and the recovered production permeability being37%.

Following this test, a mixture of 5% HCl in 1.34 g/ml CaBr₂ was loadedin the chamber for 6 hours--as described in Test 2. Recoveredpermeabilities following this treatment were 23% in the injectiondirection and 54% in the production direction.

Test 5

A test similar to Test 4 above was run, however in the formulationincluded amine substituted polymers with the substituted starch. Theformulations were the same as those described in Test 3 usingformulation #2. Heat aging, filtercake build-up, and precursor soaks (aswell as acid soaks) were again run at 66° C. Fluid breakthrough wasnoted just after a second 3 hour soak, as fresh 2.3% t-butylhydroperoxide was started, and then stopped after the first hour (4hours total) when fluid totaled 60 ml. The injection recoveredpermeability was 4% and the production recovered permeability was 45%.Following a single 6 hour acid soak similar to that in Test 4, therecovered permeabilities were 89% in the injection direction and 72% inthe production direction.

Test 6 (for comparison to Test 2)

A test using formulation #1 was prepared, heat aging, filtercakebuild-up, and acid soaks were run at 93° C. No oxidizer or oxidativeprecursor was included. Recovered permeability after 6 hrs. soak with a5% HCl solution in 1.34 g/ml CaBr₂ brine at 93° C. was less than 50% inboth directions. After a second 6 hr. treatment with fresh acidsolutions, the recovered permeability was 92% in the injection directionand 90% in the production direction.

Test 7 (also for comparison to Test 2)

A test using formulation #2 was prepared, heat aging, filtercakebuild-up, and acid soaks were run at 93° C. No oxidizer or oxidativeprecursor was run. Recovered permeability after 6 hrs. soak with a 5%HCl solution in 1.34 g/ml CaBr₂ brine at 93° C. was less than 50% inboth directions. After a second 6 hr. treatment with fresh acidsolutions, the recovered permeability was 88% in the injection directionand 93% in the production direction.

Test #8 (for comparison to #1 and 3)

A filtercake as described above was prepared using formulation #1 at 38°C. for heat aging and permeability testing. Two soaks of 6 hours eachwere run using 5% HCl in 1.34 g/ml CaBr₂ brine at room temperature and3,500 kPa differential pressure. After the second soak, some acid wasdisplaced through the Aloxite disk and permeabilities were tested. Thesewere essentially zero in both directions. The cell was loaded again with5% HCl in 1.34 g/ml CaBr₂ brine and a third 6 hour soak repeated.Recovered injection permeability was 81% and recovered productionpermeability was 90%.

The foregoing description is illustrative and explanatory of preferredembodiments of the invention, and variations in the size, shape,materials and other details will become apparent to those skilled in theart. It is intended that all such variations and modifications whichfall within the scope or spirit of the appended claims be embracedthereby.

What is claimed is:
 1. A method for removing filtercake from asubterranean borehole, comprising:drilling the borehole with a drillingfluid to form a filtercake comprising an oxidation degradable component;contacting the filtercake with a fluid containing a soluble organichydroperoxide stable at ambient temperature and thermally activatable atthe downhole temperature of the filtercake; allowing the organichydroperoxide to remain at the downhole temperature for a period of timeeffective to form an oxidizing agent to degrade the oxidation degradableagent and decompose the filtercake; flushing away the decomposedfiltercake.
 2. The method of claim 1 wherein the oxidation-degradablecomponent is a polymer.
 3. The method of claim 2 wherein the polymercomprises starch, cellulose or xanthan.
 4. The method of claim 1 whereinthe organic hydroperoxide is t-butylhydroperoxide and the boreholetemperature is at least about 80° C.
 5. The method of claim 1 whereinthe organic hydroperoxide is selected from the group consisting ofcumene hydroperoxide, t-butyl dihydroperoxide, and amylhydroperoxide,and the borehole temperature is at least about 80° C.
 6. The method ofclaim 1 wherein an activating agent for the organic hydroperoxide isflushed over the filtercake prior to contacting the filtercake with theorganic hydroperoxide.
 7. The method of claim 1 wherein the filtercakecomprises an activating agent for activating the organic hydroperoxide.8. The method of claim 7 wherein the activating agent comprises anascorbic acid.
 9. A method for removing filtercake from a subterraneanborehole, comprising:drilling the borehole with a drilling fluid to forma filtercake comprising an oxidation degradable component and anactivating agent for activating a precursor of an oxidizing agent,wherein the filtercake is free of the precursor; contacting thefiltercake with a fluid containing the precursor of the oxidizing agent,wherein the precursor is soluble in the fluid, stable at ambienttemperature and thermally activatable at the downhole temperature of thefiltercake to form the oxidizing agent; allowing the precursor to remainin contact with the filtercake to form the oxidizing component todegrade the oxidation degradable agent and decompose the filtercake;flushing away the decomposed filtercake.
 10. The method of claim 9wherein the oxidation-degradable component is a polymer.
 11. The methodof claim 10 wherein the polymer comprises starch, cellulose or xanthan.12. The method of claim 9 wherein the precursor is selected from thegroup consisting of t-butyl hydroperoxide, cumene hydroperoxide, t-butyldihydroperxoide, and amylhydroperoxide.
 13. The method of claim 9further comprising the steps ofinstalling gravel pack screens and toolassemblies into the borehole prior to the step of contacting thefiltercake with the precursor; determining fluid losses; and adding sandin a nonviscosified carrier to the borehole.
 14. A method for removingfiltercake from a subterranean borehole, comprising:drilling theborehole with a drilling fluid to form a filtercake comprising anoxidation degradable component and an activating agent comprising atertiary amine for activating an organic hydroperoxide; thereaftercontacting the filtercake with a fluid containing a solution of theorganic hydroperoxide; allowing the organic hydroperoxide to remain atthe downhole temperature in contact with the activating agent in thefiltercake for a period of time effective to form an oxidizing agent todegrade the oxidation degradable component and decompose the filtercake;flushing away the decomposed filtercake.
 15. The method of claim 14wherein the tertiary amine comprises triethanolamine.
 16. The method ofclaim 15 wherein the tertiary amine comprises diethylaminoisopropanol.17. The method of claim 14 wherein the activating agent is a polymercomprising pendant tertiary amine moieties.
 18. The method of claim 14wherein the oxidation degradable component is functionalized with theamine moieties.
 19. A method for removing filtercake from a subterraneanborehole, comprising:drilling the borehole with a drilling fluid to forma filtercake comprising an oxidation degradable component and anactivating agent for activating an organic hydroperoxide; flushing overthe filtercake with an additional activating agent comprising a metalsalt solution wherein the metal is selected from the group consisting ofcopper, cobalt, nickel, chromium, iron and combinations thereof;thereafter contacting the filtercake with a fluid containing a solutionof the organic hydroperoxide; allowing the organic hydroperoxide toremain at the downhole temperature in contact with the activating agentsin the filtercake for a period of time effective to form an oxidizingagent to degrade the oxidation degradable component and decompose thefiltercake; flushing away the decomposed filtercake.
 20. A method forremoving filtercake from a subterranean borehole, comprising the stepsof:drilling the borehole with a drilling fluid to form a filtercakecomprising an oxidation degradable component and an activating agent foractivating a precursor of an oxidizing agent, wherein the filtercake isessentially free of the precursor; installing gravel pack screens andtool assemblies into the borehole; determining fluid losses; adding sandin a non-viscosified carrier to the borehole after the screeninstallation step, contacting the filtercake with the precursor of theoxidizing agent; allowing the precursor to remain in contact with thefiltercake for a period of time effective to form the oxidizing agent todegrade the oxidation degradable component and decompose the filtercake;flushing away the decomposed filtercake.
 21. A method for removingfiltercake from a subterranean borehole, comprising:drilling theborehole with a drilling fluid to form a filtercake comprising anoxidation degradable component and an activating agent comprising atertiary amine for activating a precursor of an oxidizing agent, whereinthe filtercake is essentially free of the precursor; contacting thefiltercake with the precursor of the oxidizing agent; allowing theprecursor to remain in contact with the filtercake to be activated bythe tertiary amine and form the oxidizing agent to degrade the oxidationdegradable component and decompose the filtercake; flushing away thedecomposed filtercake.
 22. The method of claim 21 wherein the tertiaryamine comprises triethanolamine.
 23. The method of claim 21 wherein theactivating agent is a polymer comprising pendant tertiary aminemoieties.
 24. The method of claim 23 wherein the oxidation-degradablepolymer is functionalized with the amine moieties.
 25. The method ofclaim 21 wherein the precursor is selected from alkali metal andammonium salts of persulfates, permanganates and percarbonates.